Catalytic converters are well known for the control of nitrogen oxide, hydrocarbon and carbon monoxide emissions from automobiles and trucks having gasoline engines. The exhaust gas is flowed past a solid material, in the form of a pellet or a honeycomb, which has been coated with a catalyst. The offensive gases diffuse to the catalyst and are catalytically converted to non-offensive or less-offensive gases such as water vapor, nitrogen and carbon dioxide. Such catalytic converters are not effective in controlling emissions from automobiles and trucks having Diesel engines because the exhaust from a Diesel engine contains much more soot and excess oxygen than the exhaust from a gasoline engine.
Exhaust filters have been developed for Diesel engines. For example, U.S. Pat. No. 5,098,455, herein fully incorporated by reference, disclosed a regenerable exhaust gas filter comprising an acicular mullite filter media. The exhaust gas was passed through the acicular mullite filter media to trap the soot particles. Periodically, the trapped soot particles are ignited to regenerate the filter.
As reported by Corning, a nitrogen oxide adsorber unit and Diesel oxidation unit have been used downstream of a soot filter unit to control soot, nitrogen oxides and hydrocarbon emissions from a Diesel engine (Johnson, T., Developing Trends—Diesel Emission Control Update, Aug. 7, 2001). A general treatment of the subject of diesel exhaust filters is given by Heck and Farrauto in the text book entitled CATALYTIC AIR POLLUTION CONTROL—COMMERCIAL TECHNOLOGY, 2002, ISBN0-471-43624-0 and especially in Chapters 8 and 9 thereof.
United States Patent Application Publication US 2001/0032459 A1, disclosed a Diesel exhaust gas purification system having a rigid porous wall filter element coated with a mixture of a precious metal catalyst and a nitrogen oxide absorber, such that when exhaust gas from a Diesel engine is flowed through the rigid porous wall under normal operating conditions the soot in the exhaust gas is trapped within the rigid porous wall and catalytically oxidized to carbon dioxide while the nitrogen oxide is catalytically oxidized to NO2, which NO2 is then absorbed by the nitrogen oxide absorbent. The system of the '459 patent publication is regenerated by making the exhaust gas contain excess hydrocarbon and carbon monoxide so that the absorbent is regenerated by releasing NOx, which NOx and the remaining hydrocarbon and carbon monoxide are catalytically converted to nitrogen and carbon dioxide.
The system of the '459 patent publication was an important advance in the art but the porous wall material used (cordierite) is less porous than desired (requiring a larger unit to control back pressure) and the overall cost effectiveness of a device made according to the '459 patent publication was less than desired.